Phase noise of signal sources may be a severe cause of performance degradation in communication systems. Thus, in the production stage of radio frequency (RF) chips that are configured to handle electrical, optical and/or other electro-magnetic signals in the radio frequency range (3 kHz up to 300 THz), it is important to accurately measure any phase noise present in an output signal of the device under test (DUT), while keeping the cost of test (COT) as low as possible.
Exemplary delay-line discriminator techniques for phase noise measurements can offer an advantage of avoiding external sources with good phase noise properties. FIG. 1 illustrates an exemplary block diagram of a delay-line discriminator according to a conventional implementation. An RF signal to be measured is split into two paths: a first path is passed through a tunable phase shifter, and a second path is passed through a delay-line. A signal at the output of the phase shifter is then mixed with a signal at the output of the delay-line. In one embodiment, it is desired that the two signals applied to the mixer are in phase quadrature. This may be obtained by tuning the phase shifter such that the output signal of the phase shifter is in phase quadrature to the output signal of the delay-line. However, for tuning the tunable phase shifter, frequent and time-consuming calibrations may be necessitated. Furthermore, such a method may be difficult to automate.
FIG. 1 illustrates an exemplary block diagram of a delay-line discriminator according to a conventional implementation. In one embodiment, as illustrated in FIG. 1, a delay-line discriminator 100 comprises a delay-line 102, a phase shifter 103, a mixer 104, and a low-pass filter 105. The delay-line discriminator 100 is adapted or configured to measure the phase noise of a test signal 110, which may be provided by a source under test 101. The test signal 110 is input to the delay-line 102 and the phase shifter 103. The delay-line 102 provides an output signal 111, which is a delayed version of the test signal. The phase shifter 103 provides an output signal 112, which is a phase-shifted version of the test signal. The delayed test signal 111 and the phase-shifted test signal 112 are input to the mixer 104, which provides a mixed signal 113. The mixed signal 113 is input to the low-pass filter 105. The low-pass filter 105 provides a low-pass filter output signal 114, which may be analyzed by further units inside the delay-line discriminator 100 or outside of the delay-line discriminator 100.
The test signal 110 may have, at least approximately, a cosine-formed shape with a radian frequency ω0 and a time-depending phase Φ(t), for example having the form cos(ω0t+Φ(t)). The delayed test signal 111 is a delayed version of the test signal 110, e.g. having the form cos(ω0(t−Td)+Φ(t−Td)). The phase-shifted test signal 112 is a phase-shifted version of the test signal 110, for example with a phase shift φ, having the signal form cos(ω0t+Φ(t)+φ). The delayed test signal 111 and the phase-shifted test signal 112 are related with respect to the delay Td of the delay-line 102 and the phase φ of the phase shifter 103, according to ω0Td+φ=π/2 (or ω0Td+φ=π/2+kπ, with k being an integer number). This condition necessitates that the inputs to the mixer 104, that is the delayed test signal 111 and the phase-shifted test signal 112, are in phase quadrature. The mixer 104 provides a mixed signal 113, which is input to the low-pass filter 105, with the low-pass filter 105 providing a low-pass filtered output signal 114.
In one embodiment a cosine-formed test signal 110 input into a delay-line discriminator 100 results in a low-pass filter output signal 114 having a signal form of u(t)=Φ(t)−Φ(t−Td). A power spectral density of the low-pass filter output signal u(t) may be expressed as Pu(f)=|H(f)|2PΦ(f), wherein PΦ(f) is the power spectral density of the phase noise Φ(t), and |H(f)|2 is the power transmission factor by which the power spectral density PΦ(f) of the phase noise Φ(t) is transmitted to the power spectral density of the low-pass filter output signal u(t). The transfer function H(f) from the phase noise Φ(t) to the low-pass filter output signal u(t) corresponds to H(f)=1−exp(−j2πfTd).
If the delayed test signal 111 and the phase-shifted test signal 112 are in phase quadrature, mixed cosine and sine terms in the low-pass filter output signal u(t) are cancelled, such that the power spectral density of the low-pass filter output signal u(t) is independent of the radian frequency ω0 of the test signal 110. To provide this phase quadrature property of the delayed test signal 111 and the phase-shifted test signal 112, a tunable phase shifter 103 may be necessitated. The tunable phase shifter 103 has to be adjusted for each input frequency, making the calibration process time consuming and not well indicated for automatic measurements.
In view of the above, there is a need to find a phase noise measurement concept which brings along a sufficient accuracy without the need of complex calibration. Further, it is desirable to have an improved phase noise measurement concept which retains the advantages of using a phase noise discriminator.